An exclusive YouGov poll for Channel 4 News has challenged assumptions about the effect on the electoral fortunes of all three main political parties of adopting the Alternative Vote system.

Please wait while this video loads. If it doesn't load after a few seconds you may need to have Adobe Flash installed.

An exclusive poll by YouGov for Channel 4 News suggests Britain's main political parties may have seriously misjudged the effect a move to the Alternative Vote system would have on their fortunes at the ballot box.

The results, which threaten to turn the debate over changes to Britain's electoral system on on its head, show that Labour would suffer in an AV election while support for the Conservatives would remain the same.

Labour leader Ed Miliband is urging voters to scrap the current First Past the Post (FPTP) system - where a parliamentary candidate who gets more votes than anyone else wins a seat - in favour of AV in the referendum on May 5.

Liberal Democrat leader Nick Clegg is also vigorously supporting a switch to AV, where candidates are ranked in order of preference, and second choices may be used to produce a winner with more than 50 per cent of the vote.

The "No to AV" campaign claims to have the support of more than 100 Labour MPs, while the vast majority of Conservatives are expected to back leader David Cameron in opposing the change.

But the new poll, conducted by YouGov president Peter Kellner, suggests the parties' long-standing assumptions about what how AV would affect their election prospects could be ill-founded. Crucially, it challenges the long-held assumption that Lib Dem voters would vote for Labour as a second preference, revealing that these votes are actually swinging to the Conservatives.

The Alternative Vote: your questions answered
Graphics

4,000 people were asked how they would vote in an online survey, using mock ballot papers tailored to reflect regional differences in England, Wales and Scotland.

The results indicate that a General Election under AV would be broadly similar to First Past the Post.

Like all current opinion polls, this one suggests that Labour would win if a snap election was called today.

But Ed Miliband's party would win 13 fewer seats under AV than FPTP - 342 rather than 355.

The Liberal Democrats would win 13 more seats while the Conservatives would win the same number, 255, under both systems.

These results suggest a major shift in voting patterns since the last time a sample of the electorate was asked how they would have voted under AV.

A survey carried out by the British Election Study at the University of Essex immediately after last year's General Election put the Tories on 283 seats (down 22), Labour on 248 (down 10) and the Lib Dems on 89 (up 32) under AV.

The latest research shows that Labour would still suffer if an AV election was called today but the Lib Dems would have far less of an advantage and the Conservatives would see no change in their power base.

Mr Kellner, who asked voters exactly the same questions as the 2010 researchers, said the key difference this time is what happens to second preferences.

Under AV, if no clear winner emerges, the candidate who comes last is eliminated and their second-preference votes are redistributed until someone gets more than 50 per cent.

The 2010 survey showed 51 per cent of Labour voters put the Lib Dems as their second choice. In today's poll the proportion is only 16 per cent.

Some 39 per cent of Lib Dem voters chose Labour as their second preference in last year's survey. That has now fallen to 24 per cent.

Now more Lib Dem voters - 31 per cent instead of 24 per cent- would be likely to support a second-choice Tory candidate this time.

Conservative voting patterns appear to have remained stable, with 41 per cent now saying their second choice would be a Lib Dem candidate and 27 per cent backing UKIP as a second preference. In 2010 the figures were 41 per cent and 23 per cent.

In both studies, fewer than 10 per cent of Labour voters said they would consider naming a Conservative as their number two candidate, and vice versa.

The shift suggests that the experience of Coalition government has made it far less likely that Labour and Lib Dem voters would consider giving each other a potential boost at the ballot box.More surprisingly, the research suggests that Conservative fears over AV could be unfounded.

'AV assumptions out of date'
"Many of the traditional assumptions about the impact of AV look as if they are out of date," YouGov's Peter Kellner told Channel 4 News.

"The truth is that if Britain changes its voting system, people and parties will change the way they act.

"Nobody can guarantee that, in the long run, any one party will always benefit or always suffer."
Cameron on AV

Effect on marginal constituencies

In a marginal Conservative constituency like Harlow in Essex, where an election is likely to be a straight fight between Labour and the Tories,

Labour would need to collect a significant number of Lib Dem second preferences to win.

The poll results suggest a greater proportion of Lib Dem voters would make the Conservatives their second choice.

And those voting UKIP, likely to be eliminated before the final run-off in a seat like Harlow, could do the same, giving the Tory candidate a more comfortable win under AV.

YouGov says that, as well as Harlow, Aberconwy, Brigg and Goole, Bristol Northwest, Crewe and Nantwich, Dudley South, Ilford North, Peterborough, Ribble South and Stourbridge are seats that the Tories might lose to Labour under FPTP, but could save under AV.

The Lib Dems might hope to save Bermondsey and Old Southwark, Bristol West and Cambridge from a swing to Labour under AV, and they are more likely to save Berwick upon Tweed, Berwickshire, Brecon and Radnor, Carshalton and Wallington, Cheltenham, Devon North, Eastleigh, Kingston and Surbiton, Portsmouth South and Torbay from the Tories.

Clwyd West is a seat that might remain Conservative under FPTP, but be won by Labour under AV, according to YouGov.

The company came up with the list of seats after assuming a uniform swing across the country, and assuming that people's second preferences in individual seats would be the same across the country - so a Lib Dem voter in London would have the same second preferences as a Lib Dem voter in Burnley.

The seats are mostly those where a swing of 6 per cent or so still leaves Labour and Conservative very close, so a small number of reallocated second or third preferences make the difference.